Converters for adapting larger gauge guns for shooting smaller gauge ammunition shells have long been known, such as taught by Mayer U.S. Pat. No. 1,179,021 issued Apr. 11, 1916, including disposable adapters made of plastic as disclosed in Bumiller U.S. Pat. No. 3,078,800 issued Feb. 26, 1963. A common adapter for converting 12 gauge shotguns to .410 gauge which is available on the market and comprises a metal tube several times the length of a 12 gauge shotgun shell, is disclosed in Knode Jr. et al. U.S. Pat. No. 3,339,304 issued Sept. 5, 1967.
These prior art converters or adapters are heavy and bulky, or not reusable, plus the fact that they are not adaptable to all types of actions, and particularly pump and semi-automatic guns.